1. Field of the Invention
This invention relates to microstrip transmission lines and microstrip transmission line components operating in the millimeter wave region of the frequency spectrum and more particularly to a microstrip resonance isolator for use with such microstrip transmission lines and microstrip components.
2. Description of the Prior Art
Isolators are essentially two port, non-reciprocal attenuation devices which are used in RF transmission line applications, such as in the millimeter wave region of the frequency spectrum, for example, to provide a low loss transmission of electromagnetic wave energy from the input port to the output port but only a very limited or attenuated transmission of energy from the output port to the input port. They are often used in applications, such as in radar equipment, for example, to protect signal sources from other undesirable or damaging signal sources. Resonance isolators which utilize a thin substrate of a hexagonal, grain-oriented ferrite material have been developed which rely on the absorption of energy that occurs at ferromagnetic resonance. A ferrite resonance isolator of this type is shown and described in U.S. Pat. No. 4,459,567 which was issued July 10, 1984 to the applicants of the present application and was assigned to the assignee of the present application.
Planar type circuitry using microstrip is widely used in millimeter wave frequency applications because it permits the design of equipment having extremely small size and low weight which is desirable for many items of military and commercial equipment, such as the aforementioned radar equipment, for example. Unfortunately, resonance isolators utilizing hexagonal ferrite materials which are suitable for use in the millimeter wave region of the frequency spectrum are not available for microstrip transmission line applications at the present time. The resonance isolator shown and described in said U.S. Pat. No. 4,459,567 is designed in the solid, dielectric waveguide medium. Although circulators are often employed in millimeter wave microstrip applications as isolator devices, the circulators are more lossy and bulky than ferrite resonance isolators. Additionally, the circulators often require biasing magnets for operation which increases the size and weight of the equipment in which the circulator is installed. Accordingly, a need exists for a resonance isolator which will operate in microstrip applications in the millimeter wave region of the frequency spectrum.